heavy oil and tar sand,crude oils below 20° API gravity are usually considered to be heavy. The lighter conventional crudes are often waterflooded to enhance recovery. The injection of water into the reservoir helps to maintain reservoir pressure and displace the oil toward the production wells. In general, waterflooding is most effective with light crude oils of API gravity 25° and higher and becomes progressively less effective with oils below 25° API. With crudes of 20° and lower, waterfloods are essentially ineffective and thermal recovery becomes necessary. Very few thermal projects are successful in recovering oil of less than 10° API gravity. Heavy crude ... (100 of 3,301 words)